xml
  <?xml version="1.0" encoding="utf-8" ?> 
<x:styleSheet xmlns:x="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
<x:fonts count="1">
 <x:font>
  <x:sz val="11" /> 
  <x:color theme="1" /> 
  <x:name val="Calibri" /> 
  <x:family val="2" /> 
  <x:scheme val="minor" /> 
  </x:font>
  </x:fonts>
<x:fills count="2">
 <x:fill>
  <x:patternFill patternType="none" /> 
  </x:fill>
  </x:fills>
 <x:borders count="1">
 <x:border>
  <x:left /> 
  <x:right /> 
  <x:top /> 
  <x:bottom /> 
  <x:diagonal /> 
  </x:border>
  </x:borders>
 <x:cellXfs count="2">
  <x:xf numFmtId="0" fontId="0" fillId="0" borderId="0" /> 
 <x:xf numFmtId="0" fontId="0" fillId="0" borderId="0" applyAlignment="1">
  <x:alignment wrapText="1" /> 
  </x:xf>
  </x:cellXfs>
  </x:styleSheet>

Code
 private void InitializeStyleSheet()
        {
            spreadSheet.WorkbookPart.AddNewPart<WorkbookStylesPart>();
            spreadSheet.WorkbookPart.WorkbookStylesPart.Stylesheet = new Stylesheet();
            Stylesheet stylesheet = spreadSheet.WorkbookPart.WorkbookStylesPart.Stylesheet;

            stylesheet.Fonts = new DocumentFormat.OpenXml.Spreadsheet.Fonts(new Font(new FontSize() { Val = 11D }, new Color() { Theme = (UInt32Value)1U },
                                        new FontName() { Val = "Calibri" }, new FontFamily() { Val = 2 },
                                        new DocumentFormat.OpenXml.Spreadsheet.FontScheme() { Val = FontSchemeValues.Minor })) { Count = (UInt32Value)1U };
            stylesheet.Fills = new Fills(new DocumentFormat.OpenXml.Spreadsheet.Fill(new DocumentFormat.OpenXml.Spreadsheet.PatternFill() { PatternType = PatternValues.None })) { Count = (UInt32Value)2U };
            stylesheet.Borders = new Borders(new Border(new DocumentFormat.OpenXml.Spreadsheet.LeftBorder(),
                new DocumentFormat.OpenXml.Spreadsheet.RightBorder(), new DocumentFormat.OpenXml.Spreadsheet.TopBorder(),
                new DocumentFormat.OpenXml.Spreadsheet.BottomBorder(), new DiagonalBorder())) { Count = (UInt32Value)1U };

            stylesheet.CellFormats = new CellFormats();
            stylesheet.CellFormats.Count = 2;
            CellFormat cf0 = stylesheet.CellFormats.AppendChild(new CellFormat());
            cf0.NumberFormatId = 0;
            cf0.FontId = 0;
            cf0.BorderId = 0;
            cf0.FillId = 0;
            CellFormat cf = stylesheet.CellFormats.AppendChild(new CellFormat());
            cf.Alignment = new Alignment();
            cf.ApplyAlignment = true;
            cf.NumberFormatId = 0;
            cf.FontId = 0;
            cf.BorderId = 0;
            cf.FillId = 0;
            cf.Alignment.WrapText = true;
            spreadSheet.WorkbookPart.WorkbookStylesPart.Stylesheet.Save();
        }

Code
   public void WriteNewCell(Row row, string text, string cellName)
        {
            int index = InsertSharedStringItem2(text, shareStringPart);
            Cell cell = new Cell() { CellReference = new StringValue(cellName) };
            cell.CellValue = new CellValue(index.ToString());
            cell.DataType = new EnumValue<CellValues>(CellValues.SharedString);
            cell.StyleIndex = 1;
            row.Append(cell);
        }